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TRANSCRIPT
Colonel Frank Seely School
Exampro A-level Physics (7407/7408) 3.2.1.5 Classification of particles
Name:
Class:
Author:
Date:
Time: 151
Marks: 130
Comments:
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Colonel Frank Seely School
Q1.Leptons, mesons and baryons are three classes of sub-atomic particles.
(a) Some classes of particles are fundamental; others are not. Circle the correct category for each of these three classes.
leptons fundamental/not fundamental
mesons fundamental/not fundamental
baryons fundamental/not fundamental
(1)
(b) Name the class of particles of which the proton is a member.
........................................................................................................................(1)
(c) By referring to the charges on up and down quarks explain how the proton has a charge of +1e.
........................................................................................................................
........................................................................................................................
........................................................................................................................(2)
(Total 4 marks)
Q2. The list of sub-atomic particles below contains particles that are either hadrons or leptons:
electron muon neutrino neutron pi-meson proton
(a) Complete the table below by adding the names of the particles to the correct box.
Hadrons
Leptons
(4)
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(b) Underline the names of the particles that are baryons.(2)
(Total 6 marks)
Q3. (a) A particle is made up from an anti-up quark and a down quark.
(i) Name the classification of particles that has this type of structure.
...............................................................................................................(1)
(ii) Find the charge on the particle.
(1)
(iii) State the baryon number of the particle.
(1)
(b) A suggested decay for the positive muon (µ+) is
µ+ → e+ + ve
Showing your reasoning clearly, deduce whether this decay satisfies the conservation rules that relate to baryon number, lepton number and charge.
Baryon number ............................................................................................
Lepton number .............................................................................................
Charge ...........................................................................................................(3)
(Total 6 marks)
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Q4. (a) State the quark substructure of a neutron.
........................................................................................................................(2)
(b) Circle the terms below that can be used to describe a neutron.
antiparticle baryon fundamental particle hadron lepton meson(2)
(Total 4 marks)
Q5. Neutrons were discovered when beryllium, Be, was bombarded with alpha particles.
An alpha particle knocked a neutron out of a beryllium nucleus producing a carbon nucleus, C.
(a) Write down the equation that describes this reaction.
........................................................................................................................(2)
(b) (i) Describe the quark substructure of a neutron.
...............................................................................................................
...............................................................................................................(1)
(ii) Describe how the quark substructure of a meson differs from that of a baryon such as a neutron.
...............................................................................................................
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(1)(Total 4 marks)
Q6. State the differences in quark structure between a meson and a baryon.
.................................................................................................................................
.................................................................................................................................
.................................................................................................................................
.................................................................................................................................(Total 2 marks)
Q7.(a) Name two hadrons.
........................................................................................................................
........................................................................................................................
(b) Name two leptons which are also antiparticles.
........................................................................................................................
........................................................................................................................
(c) State a possible quark structure of the pion π0.A table of the properties of quarks is given in the Data booklet.
........................................................................................................................
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(d) A K– kaon is a strange particle.State one characteristic of a strange particle.
........................................................................................................................
........................................................................................................................(Total 4 marks)
Q8.(a) State whether or not each of the following properties of a baryon is conserved when it decays by the weak interaction.
charge .................................................................................................................
baryon number ...................................................................................................
strangeness .......................................................................................................(2)
(b) State, with a reason, whether or not each of the following particle reactions is possible.
(i) p + π– → K– + π+
...............................................................................................................
...............................................................................................................
...............................................................................................................
(ii) p + → n = e+
...............................................................................................................
...............................................................................................................
...............................................................................................................(4)
(Total 6 marks)
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Colonel Frank Seely School
Q9.(a) (i) How do hadrons differ from all other subatomic particles?
...............................................................................................................
...............................................................................................................
(ii) Give the quark composition of the following particles.
neutron .................................................................................................
neutral pion ...........................................................................................
...............................................................................................................
(iii) Classify the following as either leptons, baryons or mesons.
kaon ......................................................................................................
muon .....................................................................................................(5)
(b) Which is the most stable baryon?
........................................................................................................................(1)
(c) This table may be useful in answering the questions which follow.
particle baryon number
lepton number strangeness
π– 0 0 0
p 1 0 0
p –1 0 0
e– –1 0 0
e+ 0 1 0
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Colonel Frank Seely School
νe 0 –1 0
The particle X, which is a strange particle, decays in the following way:
X → π– + p
(i) State whether X is a meson, a baryon or a lepton.
...............................................................................................................
(ii) Use conservation laws to decide whether each of the following decays of the π– is possible. Give a reason for your answer.
(A) π– → e+ + νe
Is this decay possible? ........................................................
reason ...................................................................................................
(B) π– → p + e– + e+
Is this decay possible? ........................................................
reason ...................................................................................................(5)
(Total 11 marks)
Q10. (a) Quarks may be combined together in a number of ways to form sub-groups of hadrons. Name two of these sub-groups and for each, state its quark composition.
sub-group 1 ..................................................................................................
......................................................................................................................
sub-group 2 ..................................................................................................
......................................................................................................................(3)
(b) A free neutron is an unstable particle.
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(i) Complete the following to give an equation that represents the decay of a neutron.
n →
(ii) Describe the change that occurs to the quark structure when a neutron decays.
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................(4)
(Total 7 marks)
Q11. Some subatomic particles are classified as hadrons.
(a) What distinguishes a hadron from other subatomic particles?
......................................................................................................................
......................................................................................................................(1)
(b) Hadrons fall into two subgroups. Name each subgroup and describe the general structure of each.
subgroup 1 ...................................................................................................
......................................................................................................................
subgroup 2 ...................................................………......................................
......................................................................................................................(3)
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Colonel Frank Seely School
(c) The following equation represents an event in which a positive muon collides with a neutron to produce a proton and an antineutrino.
n + μ+ p + .
Show that this equation obeys the conservation laws of charge, lepton number and baryon number.
......................................................................................................................
......................................................................................................................
......................................................................................................................(3)
(Total 7 marks)
Q12. The equation
p n + β+ + ve
represents the emission of a positron from a proton.
(a) Energy and momentum are conserved in this emission.What other quantities are conserved in this emission?
......................................................................................................................
......................................................................................................................
......................................................................................................................
......................................................................................................................(3)
(b) Draw the Feynman diagram that corresponds to the positron emission represented in the equation.
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(4)
(c) Complete the following table using ticks and crosses .
particle fundamental particle meson baryon lepton
p
n
β+
ve
(4)(Total 11 marks)
Q13. (a) (i) Give an example of an exchange particle other than a W+ or W– particle, and state the fundamental force involved when it is produced.
exchange particle ................................................................................
fundamental force ...............................................................................
(ii) State what roles exchange particles can play in an interaction.
.............................................................................................................
.............................................................................................................
.............................................................................................................(4)
(b) From the following list of particles,
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p e+ μ– π0
identify all the examples of
(i) hadrons, .............................................................................................
(ii) leptons, …...........................................................................................
(iii) antiparticles, .......................................................................................
(iv) charged particles. ...............................................................................(4)
(Total 8 marks)
Q14. Leptons, mesons and baryons are three classes of sub-atomic particles.
(a) Some classes of particles are fundamental; others are not. Circle the correct category for each of these three classes.
Leptons fundamental/not fundamentalMesons fundamental/not fundamentalBaryons fundamental/not fundamental
(1)
(b) Name the class of particles of which the proton is a member.
......................................................................................................................(1)
(c) By referring to the charges on up and down quarks, explain how the proton has acharge of + 1e.
......................................................................................................................
......................................................................................................................(2)
(Total 4 marks)
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Colonel Frank Seely School
Q15. (a) Give the name of a particle that is a hadron.
......................................................................................................................(1)
(b) Pions are mesons.
Give a possible quark structure for a pion.
......................................................................................................................(1)
(Total 2 marks)
Q16. (a) (i) Name two baryons.
.............................................................................................................(2)
(ii) State the quark structure of the pion .
.............................................................................................................(1)
(b) (i) The K+ kaon is a strange particle. Give one characteristic of a strange particle that makes it different from a particle that is not strange.
.............................................................................................................
.............................................................................................................(1)
(ii) One of the following equations represent a possible decay of the K+ kaon.
K+ → π+ + π0
K+→ μ+ + State, with a reason, which one of these decays is not possible.
.............................................................................................................
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.............................................................................................................(2)
(c) Another strange particle, X, decays in the following way:
X → π– + p(i) State what interaction is involved in this decay.
.............................................................................................................(1)
(ii) Show that X must be a neutral particle.
.............................................................................................................
.............................................................................................................(1)
(iii) Deduce whether X is a meson, baryon or lepton, explaining how you arrive at your answer.
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................(2)
(iv) Which particle in this interaction is the most stable?
.............................................................................................................(1)
(Total 11 marks)
Q17. (a) The table gives information about some fundamental particles.
Complete the table by filling in the missing information.
particle quark charge strangene baryon
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structure number
uud 0
Sigma + uus + 1
ud 0 0
(7)
(b) Each of the particles in the table has an antiparticle.
(i) Give one example of a baryon particle and its corresponding antiparticle.
particle ..................................................................................................
antiparticle ............................................................................................(1)
(ii) State the quark structure of an antibaryon.
...............................................................................................................(1)
(iii) Give one property of an antiparticle that is the same for its corresponding particle and one property that is different.
Same ....................................................................................................
...............................................................................................................
Different ................................................................................................
...............................................................................................................(2)
(Total 11 marks)
Q18.Mesons that contain a strange (or antistrange) quark are known as K-mesons or kaons. Mesons are a sub-group of a larger group of particles.
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(a) (i) State the name of this larger group of particles.
...............................................................................................................(1)
(ii) Determine the charge on a kaon with a quark structure of us.
...............................................................................................................(1)
(b) A proposed decay for this kaon is
(i) Apply the law of conservation of strangeness to the proposed decay.
...............................................................................................................
...............................................................................................................
...............................................................................................................(1)
(ii) Comment on whether or not this decay is possible.
...............................................................................................................
...............................................................................................................
...............................................................................................................(1)
(Total 4 marks)
Q19.Which of the following is not true?
A Each meson consists of a single quark and a single
antiquark.
B Each baryon consists of three quarks.
C The magnitude of the charge on every quark is
D A particle consisting of a single quark has not been
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observed.
(Total 1 mark)
Q20.What are the numbers of hadrons, baryons and mesons in an atom of 73Li?
hadrons baryons mesons
A 7 3 3
B 7 4 4
C 7 7 0
D 10 7 0
(Total 1 mark)
Q21.(a) Complete the table comparing some of the properties of the positive pion, π+, and the proton.
Name π+ Proton
Relative charge +1
Baryon number
Quarkcomposition
(5)
(b) When a positive pion interacts with a proton, a kaon can be produced, along with another strange particle, as shown in this equation
Circle the type of interaction shown in this equation.
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Electromagnetic Gravitational Strong Nuclear Weak Nuclear
(1)
(c) Deduce the relative charge, baryon number and strangeness of particle X.
(3)
(d) Particle X can decay to produce a neutron and positive pion as shown in this equation
Circle the type of interaction shown in this equation.
Electromagnetic Gravitational Strong Nuclear Weak Nuclear
(1)
(e) Explain your answer.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................(2)
(f) The neutron and positive pion will then decay. The positive pion can decay into a positron and an electron neutrino.
Write down the equation for the decay of the neutron.
(2)
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(g) Explain why no further decays occur.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................(2)
(Total 16 marks)
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M1.(a) lepton fundamental meson, baryon not fundamental allow underline or crossing out wrong options
B1(1)
(b) (i) baryon / hadronB1
(1)
(ii) u u dB1
= +1(e)B1
(2)[4]
M2. (a) all correct (- 1 for each mis-classification or omission;total not to go below zero)hadrons: : proton/neutron/pionleptons: electron/muon/neutrino
B44
(b) proton
B1Neutron(-1 for each mis-classification; total not to go below zero)
B12
[6]
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Colonel Frank Seely School
M3. (a) (i) meson (not muon)
B11
(ii) –1 or –1.6 × 10–19 C or –e
B11
(iii) 0
B11
(b) baryon number 0 → 0 + 0 (satisfied or cs)(allow statement that as these are all leptons baryonnumber is not relevant owtte)
B1
lepton number –1 → –1 + 1 × or not satisfied
B1
charge (+)1 → (+)1 + 0 (satisfied or cs)
B13
[6]
M4. (a) Three quarks mentioned; at least one u, one d
C1
udd A12
(b) hadron
B1
Baryon
B12
[4]
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Colonel Frank Seely School
M5. (a) Be + α C + n
(condone N; any other symbol must be defined as a neutron)
B1
(Condone other symbols if Z and A correct)
B12
(b) (i) udd (1 up quarks and 2 down quarks)
B11
(ii) A meson has only two quarks(whereas a baryon has three)
B11
[4]
M6. meson has 2 quarks; baryon has 3 quarks/3 antiquarks
B1
good extra detail
B1[2]
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Colonel Frank Seely School
M7.(a) any two hadrons e.g. proton, neutron, pion, kaon, etc. (1)
(b) any two antiparticle leptons e.g. anti-(electronic) neutrino etc (1)
(c)
(d) usually created in pairs (*)normally decays into combinations of π, p and n (*)contains at least one strange quark (*)usually decays via the weak interaction (*)half - life is relatively long compared with half -life of typical particle decaying via strong interaction (*)(*) any one (1)
[4]
M8.(a) charge – yes*
baryon number – yes*
strangeness – no*
* all correct (1) (1)
deduct one for each incorrect answer(max 2)
(b) (i) no (1)
strangeness [or baryon number] not conserved (1)
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Colonel Frank Seely School
(ii) yes (1)
charge and baryon number conserved (1)(4)
[6]
M9.(a) (i) hadrons (are not fundamental) are composed of quarks [or hadrons may interact through the strong nuclear force (as well as all the other interactions)] (1)
(ii) (neutron) udd (1)(neutral pion) (1)
(iii) (kaon) meson (1)(muon) lepton (1)
5
(b) proton (1)1
(c) (i) (X) baryon (1)
(ii) (a) not possible (1) charge not conserved (1) (allow C.E. from previous line)
(b) not possible (1) baryon number not conserved (1) (allow C.E. from previous line)
5[11]
M10. (a) baryon qqqantibaryon qqqmeson qq
two names (1)composition of each sub-group (1) (1)
3
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Colonel Frank Seely School
(b) (i) n → p (1) + (1) + (e) (1)
(ii) a down (d) quark changes to an up (u) quark[or udd changes to uud] (1)
4[7]
M11. (a) hadrons are subject to the strong nuclear force[or hadrons consist of quarks (or antiquarks)] (1)
1
(b) (i) baryons and mesons (1)
baryons consist of three quarks antibaryons consist of three antiquarksmesons consist of a quark and an antiquark (any two) (1) (1)
3
(c) Q: 0 + 1 = 1 + 0 (1)L: 0 – 1 = 0 – 1 (1)B: 1 + 0 = 1 + 0 (1)
3[7]
M12. (a) baryon numberlepton numberchargestrangeness (any three) (1) (1) (1)
3
(b) Feynman diagram to show:p changing to n (1)W+ (1)ß+ and ve (1)correct overall shape with arrows (1)
4
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(c)
particle fundamentalparticle
meson baryon lepton
p × ×
n × ×
ß+ × ×
ve × ×
(1) (1) (1) (1) (one for each correct line)4
[11]
M13. (a) (i) Z0 with the weak interactiongluons or pions with the strong nuclear forceγ photons with electromagnetic interactiongravitons with gravity(any exchange particle (1) and corresponding interaction (1))
(ii) transfers energytransfers momentumtransfers force(sometimes) transfers charge any two (1)(1)
4
(b) p π0 (1)
Vee+µ− (1)
e+ (1)
pe+µ− (1)4
[8]
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M14. (a) lepton fundamental
meson, baryon not fundamental
allow underline or crossing out wrong options (1)1
(b) (i) baryon/hadron (1)
(ii) u u d (1)
+ + – = + 1(e) (1)3
[4]
M15. (a) one named hadron or obvious symbol
B11
(b) d /u /u / u or words
B11
[2]
M16. (a) (i) any two eg proton, neutron 2
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(ii) 1
(b) (i) contains a strange quark
or longer half life than expected
or decays by weak interaction 1
(ii) the second one is not possible
because lepton number is not conserved 2
(c) (i) weak (interaction) 1
(ii) mention of charge conservation
or charge conservation demonstrated by numbers 1
(iii) X must be a baryon
baryon number on right hand side is +1 2
(iv) proton/p 1
[11]
M17. (a)
particle quark structure
charge strangeness baryon number
proton uud + 1 0 1
sigma+ uus +1 -1 1
π+ ud +1 0 0
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(b) (i) examples:proton, antiquarks
1
(ii) consists of 3 antiquarks 1
(iii) same (rest) mass (energy)
difference eg baryon number/charge 2
[11]
M18.(a) (i) hadrons
B11
(ii) +1e
B11
(b) (i) (Strangeness) 1 → 0 + 0
B11
(ii) (Strangeness not conserved but) decay possible because it is a weak decay
B11
[4]
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M19.C[1]
M20.C[1]
M21.(a)
1✓
0✓ 1✓
...
ud✓ uud✓
1 mark each5
(b) Strong nuclear circled✓1
(c) Charge 1 + 1 = 1 + X X = 1✓1
Baryon number 0 + 1 = 0 + X X = 1✓1
Strangeness 0 + 0 = 1 + X X = −1✓1
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Colonel Frank Seely School
Any order
(d) Weak nuclear circled✓1
(e) Strangeness of X is -1,First mark is for showing that strangeness changes
The strangeness of the pion and neutron are both zero1
The strangeness changes from -1 to 0✓
This can only occur in weak interactions. ✓Second is for stating that this can only happen if the interaction is weak.
1
(f) . First mark is for the proton
1
n → p ✓ + β– + ve ✓Second is for the beta minus and antineutrino.
1
(g) The only particles remaining are electrons / positrons and neutrinos / antineutrinos which are stable ✓
11
And a proton which is the only stable baryon ✓1
1[16]
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E1.This question was extremely well answered with the majority of candidates gaining at least three of the four marks.
(a) A sizeable minority of candidates circled the complete opposite of all the correct answers. It is essential that candidates read the questions carefully.
(b) Credit was given for those candidates stating that the proton is a hadron.
(c) Answers were usually very clearly laid out and correct.
E2. (a) This was a high scoring question with only a handful of candidate showing a complete lack of knowledge of the classification. Electrons and the muon were the most frequent errors.
(b) Again a frequent contender for mis-classification was the muon, but there were many full marks on this sub-section.
E3. (a) (i) The majority identified that particles with this type of structure are mesons.
(ii) This should have been an easy mark using the information on the formula sheet but this was not the case for a significant proportion of the candidates who misread information from the table or made careless arithmetical errors (e.g.–2/3 – 1/3 = 1, 1/3 or –1/3).
(iii) Errors similar to those in (ii) were made in this part by a significant number of candidates.
(b) This was often well done with a clear conclusion that lepton number was not conserved. It was acceptable for candidates to state that as there were no baryons involved in the equation the conservation of baryon number did not apply. The main error in dealing with charge was to assign a charge to the neutrino.
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E4. (a) Almost all were able to indicate the correct quark substructure of the neutron. However these answers were often couched simply as, for example, ‘udd’ without any definition of these symbols leaving the examiners to infer what candidates meant.
(b) Again, many knew that the terms baryon and hadron are used to describe a neutron, but far too many also suggested that the neutron is a fundamental particle. A small group of candidates used the lepton response as an alternative to hadron.
E5. (a) Responses here were very disappointing and relatively few correct answers were seen. The question demanded only conversion of a text description into an equation but many were unable to produce an equation with the correct particles in either side of the equation. Those who could often failed to insert correct Z and A numbers.
(b) (i) The structure was known by the vast majority of the candidates.
(ii) The majority knew the difference in substructure between a baryon and a meson.
E6. Many candidates were able to suggest that mesons have two quarks and that baryons have three, but fewer were able to give good additional detail (for example, that mesons have a quark and anti-quark structure). A sizeable minority reversed the quark count in the arrangement of the two types of particle.
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E7.This question was generally done well, even though the parts of the question became progressively more difficult. Naming two hadrons was an easy task, but only the best candidates could state clearly a characteristic of a strange particle.
E8.This question, on particles and their conservation, was much more discriminating than similar questions have been in previous years. It has been common for a majority of candidates to obtain full marks in such questions. Half of the candidates did not know that strangeness is not necessarily conserved in a weak interaction.
Many errors were also seen in part (b). The most common was that charge was not thought to balance in both equations. The errors came from candidates not realising that the proton was positively charged
E9.This question again showed good discrimination and in particular showed up the weaknesses of the poorer candidates. Part (a) was usually performed well by most candidates whereas part (b) was answered incorrectly by a large majority of candidates. It was a common misconception in the answer to part (b) that the neutron was the most stable baryon.
It was common in part (c) for more than half the available marks to be earned, but often this was due to consequential errors. It was interesting to note that candidates would often work through conservation of lepton number, baryon number and strangeness but failed to consider conservation of charge. Consequently, part (c)(i)(A) was a stumbling block for most candidates.
E10. The average candidate performed well on this question. Many candidates lost marks by describing the meson as having two quarks rather than having a quark plus an antiquark. Other candidates lost marks because of the ambiguous way in which antiquarks were included in their description of a baryon.
In part (b) many of the weaker candidates were under the illusion that a lepton had a quark structure and also that the change to the quark structure during the decay was a
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change to an antiquark rather than a down quark changing to an up quark.
E11. Parts (a) and (b) posed no problem for good candidates. Some common errors made by the others included quoting a specific number of quarks making up a hadron, thereby making it a baryon or meson or replacing the correct answer of meson with a lepton or stating that a meson consisted of two quarks without specifying that one was an antiquark.
Only the top 20% of candidates gave the correct lepton numbers in part (c). Many candidates lost marks by not giving their answers in the form of full equations which included all the required numbers i.e. all the zeros were required in addition to the +1 and –1.
E12. As in the previous question, part (a) proved to be quite discriminating among the weaker candidates. A number of candidates thought mass was conserved and some also thought that the number of quarks obeyed a conservation rule.
Surprisingly few candidates obtained full marks on the Feynman diagram in part (b), the most common score being three out of the four available marks. The reason for this was a failure to show the direction of the W+ particle with either an arrow or by showing it slanting upwards.
The table in part (c) was completed successfully by the majority of candidates, the most common error being classifying both the positron and neutrino as leptons. An unfortunate ambiguity occurred in this section, in that the supplied data sheet refers to protons and neutrons as Fundamental Particles. The word ‘Fundamental’ in the data sheet was obviously not being used in its strict scientific meaning, but more as indicating important particles. Since this could easily confuse candidates it was decided to ignore the answer in the box pertaining to the proton and neutron being considered as fundamental particles.
E13. Normally the question concerning fundamental forces and particles is answered well, but this time very few candidates scored full marks. Part (a) (i) gave rise to very few problems to the prepared candidate, but in part (a) (ii), the usual answer gave only one role played by the exchange particles in the interaction, thereby losing a mark by omitting to give a second role. Another common error was to suggest that the exchange particle somehow gave energy or momentum to the interaction, rather than transferred energy or momentum.
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More able candidates had no trouble with part (b), but the less able candidates failed badly by not identifying all the examples given. The π0 particle was accepted as a possibility for an antiparticle, being its own antiparticle, but it does not appear as a required answer.
E15. Most candidates were able to answer part (a), with most common answers being proton/neutron.
Fewer candidates correctly answered part (b); here common errors included 3 quark combinations and combinations of quark-antiquarks that included the strange quark.
E16. This question was answered well and provided limited discrimination between candidates. Most were able to successfully identify two baryons and also deduce the quark structure of the pion, π+. Less able candidates found it hard to identify which of the K+ decays in part (b)(ii) were possible and they provided explanations that were not convincing.
Part (c) was answered very well with the majority able to identify the weak interaction and correctly apply charge and baryon conservation. Most candidates were well aware that the proton is the most stable baryon.
E17. Previous papers have indicated that students have a good understanding of the quark structure of hadrons and this was certainly the case in this examination. The table in part (a) was completed well and full marks were frequent. The remainder of the question was also answered well and students now seem well aware that a similarity between particles and their corresponding antiparticle is rest mass.
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Colonel Frank Seely School
E18.Part (a) (i) was answered correctly by most candidates. Part (a) (ii) was less well done with only the best candidates setting out their work in a convincing manner.
In part (b) (i) candidates often limited themselves to making statements about the conservation of strangeness without attempting to support these statements through a quantitative analysis of strangeness in the proposed decay.
Many candidates answered part (b) (ii) incorrectly thinking that charge had not been conserved or they believed that the decay was possible because strangeness was conserved. Only the highest achieving candidates were able to correctly state that the decay would be possible if it involved weak interaction.
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